Liquid-crystalline medium and liquid-crystal display

ABSTRACT

The present invention relates to dielectrically positive liquid-crystalline media comprising one or more compounds of the formula I 
     
       
         
         
             
             
         
       
     
     in which the parameters have the meaning indicated in the specification, and optionally one or more compounds selected from the group of the compounds of the formulae II and III 
     
       
         
         
             
             
         
       
     
     in which the parameters have the meaning indicated in the specification, and optionally one or more compounds of the formula IV 
     
       
         
         
             
             
         
       
     
     in which the parameters have the meaning indicated in the specification, and to liquid-crystal displays containing these media, especially to active-matrix displays and in particular to TN, IPS and FFS displays.

FIELD OF THE INVENTION

The present invention relates to liquid-crystalline media and to liquid-crystal displays containing these media, especially to displays addressed by an active matrix and in particular to displays of the twisted nematic (TN), in-plane switching (IPS) or fringe-field switching (FFS) type.

STATE OF THE ART AND PROBLEM TO BE SOLVED

Liquid-crystal displays (LCDs) are used in many areas for the display of information. LCDs are used both for direct-view displays and for projection-type displays. The electro-optical modes used are, for example, the twisted nematic (TN), super twisted nematic (STN), optically compensated bend (OCB) and electrically controlled birefringence (ECB) modes together with their various modifications, as well as others. All these modes utilise an electric field which is substantially perpendicular to the substrates or the liquid-crystal layer. Besides these modes, there are also electro-optical modes that utilise an electric field which is substantially parallel to the substrates or the liquid-crystal layer, such as, for example, the in-plane switching (IPS) mode (as disclosed, for example, in DE 40 00 451 and EP 0 588 568) and the fringe field switching (FFS) mode, in which a strong “fringe field” is present, i.e. a strong electric field close to the edge of the electrodes and, throughout the cell, an electric field which has both a strong vertical component and a strong horizontal component. These latter two electro-optical modes in particular are used for LCDs in modern desktop monitors and are intended for use in displays for TV sets and multimedia applications. The liquid crystals according to the present invention are preferably used in displays of this type. In general, dielectrically positive liquid-crystalline media having rather lower values of the dielectric anisotropy are used in FFS displays, but in some cases liquid-crystalline media having a dielectric anisotropy of only about 3 or even less are also used in IPS displays.

For these displays, novel liquid-crystalline media having improved properties are required. The addressing times in particular have to be improved for many types of application. Thus, liquid-crystalline media having lower viscosities (η), especially having lower rotational viscosities (γ₁), are required. The rotational viscosity should be 80 mPa·s or less, preferably 60 mPa·s or less and especially 55 mPa·s or less. Besides this parameter, the media must have a nematic phase range of suitable width and position and an appropriate birefringence (Δn), and the dielectric anisotropy (Δε) should be sufficiently high to allow a reasonably low operating voltage. Δε should preferably be greater than 2 and very preferably greater than 3, but preferably not greater than 15 and in particular not greater than 12, as this would prevent an at least fairly high resistivity.

The displays according to the present invention are preferably addressed by an active matrix (active matrix LCDs, AMDs for short), preferably by a matrix of thin film transistors (TFTs). However, the liquid crystals according to the invention can also advantageously be used in displays having other known addressing means.

There are numerous different display modes which use composite systems of low-molecular-weight liquid-crystal materials together with polymeric materials. These are, for example, polymer dispersed liquid crystal (PDLC), nematic curvilinearly aligned phase (NCAP) and polymer network (PN) systems, as disclosed, for example, in WO 91/05 029, or axially symmetric microdomain (ASM) systems and others. In contrast to these, the modes that are especially preferred in accordance with the present invention use the liquid-crystal medium as such, oriented on surfaces. These surfaces are typically pretreated in order to achieve uniform alignment of the liquid-crystal material. The display modes according to the present invention preferably use an electric field which is substantially parallel to the composite layer.

Liquid-crystal compositions which are suitable for LCDs and especially for IPS displays are known, for example, from JP 07-181 439 (A), EP 0 667 555, EP 0 673 986, DE 195 09 410, DE 195 28 106, DE 195 28 107, WO 96/23 851 and WO 96/28 521. However, these compositions have severe disadvantages. Amongst other deficiencies, most of them result in disadvantageously long addressing times, have inadequate values of the resistivity and/or require excessively high operating voltages.

Thus, there is a considerable need for liquid-crystalline media having suitable properties for practical applications, such as a broad nematic phase range, suitable optical anisotropy Δn corresponding to the display type used, a high Δε and particularly low viscosities.

PRESENT INVENTION

Surprisingly, it has now been found that it is possible to achieve liquid-crystalline media having a suitably high Δε, a suitable phase range and Δn which do not exhibit the disadvantages of the materials from the prior art, or at least only do so to a significantly lesser extent.

These improved liquid-crystalline media according to the present application comprise at least the following components:

-   -   a dielectrically positive component, component A, comprising         -   one or more dielectrically positive compounds of the formula             I

in which

-   R¹¹ and R¹², independently of one another, denote alkyl, alkoxy,     fluorinated alkyl or fluorinated alkoxy having 1 to 7 C atoms,     alkenyl, alkenyloxy, alkoxyalkyl or fluorinated alkenyl, alkenyloxy     or alkoxyalkyl having 2 to 7 C atoms and preferably alkyl or     alkenyl, and     -   optionally one or more compounds selected from the group of the         compounds of the formulae II and III

in which

-   R² and R³, independently of one another, denote alkyl, alkoxy,     fluorinated alkyl or fluorinated alkoxy having 1 to 7 C atoms,     alkenyl, alkenyloxy, alkoxyalkyl or fluorinated alkenyl having 2 to     7 C atoms, and R² and R³ preferably denote alkyl or alkenyl,

-   -   independently of one another, denote

-   -    preferably

-   L²¹, L²², L³¹ and L³², independently of one another, denote H or F,     L²¹ and/or L³¹ preferably denote F, -   X² and X³, independently of one another, denote halogen, halogenated     alkyl or alkoxy having 1 to 3 C atoms or halogenated alkenyl or     alkenyloxy having 2 or 3 C atoms, preferably F, Cl, —OCF₃ or —CF₃,     very preferably F, Cl or —OCF₃, -   Z³ denotes —CH₂CH₂—, —CF₂CF₂—, —COO—, trans-CH═CH—, trans-CF═CF—,     —CH₂O— or a single bond, preferably —CH₂CH₂—, —COO—, trans-CH═CH— or     a single bond and very preferably —COO—, trans-CH═CH— or a single     bond, and -   l, m, n and o, independently of one another, denote 0 or 1, and     -   optionally a dielectrically neutral component, component B,         comprising one or more compounds of the formula IV

in which

-   R⁴¹ and R⁴², independently of one another, have the meaning     indicated above for R² under formula II, preferably R⁴¹ denotes     alkyl and R⁴² denotes alkyl or alkoxy or R⁴¹ denotes alkenyl and R⁴²     denotes alkyl,

-   -   independently of one another and, in the case where

-   -    occurs twice, also these independently of one another, denote

-   -   preferably one or more of

-   -   denote(s)

-   Z⁴¹ and Z⁴², independently of one another and, in the case where Z⁴¹     occurs twice, also these independently of one another, denote     —CH₂CH₂—, —COO—, trans-CH═CH—, trans-CF═CF—, —CH₂O—, —CF₂O—, —C≡C—     or a single bond, preferably one or more of them denote(s) a single     bond, and -   p denotes 0, 1 or 2, preferably 0 or 1.

The media preferably comprise one or more dielectrically positive compounds of the formula I in which the parameters have the respective meanings indicated above and X¹ preferably denotes F.

In a preferred embodiment of the present invention, the media comprise one or more compounds of the formula I in which the parameters have the respective meanings indicated above and one of the parameters R¹¹ and R¹² denotes alkyl and the other denotes alkyl or alkenyl, preferably both denote alkyl.

The total concentration of the compounds of the formula I in the medium is preferably in the range from 1% to 50%, more preferably from 1% to 40%, even more preferably from 1% to 30%. In a particularly preferred embodiment, the total concentration of the compounds of the formula I in the medium is in the range from 2% to 20% and particularly preferably from 4% to 17%. If a single homologous compound of the formula I is used in the medium, its concentration is preferably in the range from 1% to 20%, and if two or more homologous compounds of the formula I are used in the medium, 2% to 14% of the individual homologues are preferably used.

The media according to the present invention preferably comprise one or more dielectrically positive compounds having a dielectric anisotropy of greater than 3, selected from the group of the formulae II and III.

In a preferred embodiment of the present invention, the media comprise one or more compounds selected from the group of the compounds of the formulae II-1 to II-3:

in which the parameters have the respective meanings indicated above under formula II, and in formula II-1 the parameters L²³ and L²⁴, independently of one another and of the other parameters, denote H or F, and in formula II-2

independently of one another, preferably denote

The media preferably comprise compounds which are selected from the group of the compounds of the formulae II-1 to II-3 in which L²¹ and L²² or L²³ and L²⁴ both denote F.

In a preferred embodiment, the media comprise one or more compounds selected from the group of the compounds of the formulae II-1 and II-2 in which L²¹, L²², L²³ and L²⁴ all denote F.

The media preferably comprise one or more compounds of the formula II-1, preferably selected from the group of the compounds of the formulae II-1a to II-1j:

in which the parameters have the respective meanings indicated above, and L²⁵ to L²⁸, independently of one another, denote H or F, preferably L²⁷ and L²⁸ both denote H, particularly preferably L²⁶ denotes H.

The media preferably comprise compounds selected from the group of the compounds of the formulae II-1a to II-1e in which L²¹ and L²² both denote F and/or L²³ and L²⁴ both denote F.

In a preferred embodiment, the media comprise compounds selected from the group of the compounds of the formulae II-1a to II-1h in which L²¹, L²², L²³ and L²⁴ all denote F.

Especially preferred compounds of the formula II-1 are

in which R² and X² have the meanings indicated above, and X² preferably denotes F.

The media preferably comprise one or more compounds of the formula II-2, preferably selected from the group of the compounds of the formulae II-2a to II-2c:

in which the parameters have the respective meanings indicated above, and L²¹ and L²² preferably both denote F.

The media preferably comprise one or more compounds of the formula II-3, preferably selected from the group of the compounds of the formulae II-3a to II-3e:

in which the parameters have the respective meanings indicated above, and L²⁵ and L²⁶, independently of one another and of the other parameters, denote H or F, and preferably in formulae II-3a and II-3b L²¹ and L²² both denote F, in formulae II-3c and II-3d L²¹ and L²² both denote F and/or L²³ and L²⁴ both denote F, and in formula II-3e L²¹, L²² and L²⁵ denote F.

Especially preferred compounds of the formula II-3 are

in which R² has the meaning indicated above.

In a further preferred embodiment of the present invention, the media comprise one or more compounds selected from the group of the formulae III-1 and III-2:

in which the parameters have the respective meanings indicated above under formula III.

The media preferably comprise one or more compounds of the formula III-1, preferably selected from the group of the compounds of the formulae III-1a and III-1b:

in which the parameters have the respective meanings indicated above, and the parameters L³³ and L³⁴, independently of one another and of the other parameters, denote H or F.

The media preferably comprise one or more compounds of the formula III-2, preferably selected from the group of the compounds of the formulae III-2a to III-2j:

in which the parameters have the respective meanings indicated above, and the parameters L³⁵ and L³⁶, independently of one another and of the other parameters, denote H or F.

The media preferably comprise one or more compounds of the formula preferably selected from the group of the compounds of the formulae III-1a-1 to III-1a-6:

in which R³ has the meaning indicated above.

The media preferably comprise one or more compounds of the formula III-1b, preferably selected from the group of the compounds of the formulae III-1b-1 to III-1b-4, preferably of the formula III-1b-4:

in which R³ has the meaning indicated above.

The media preferably comprise one or more compounds of the formula III-2a, preferably selected from the group of the compounds of the formulae III-2a-1 to III-2a-5:

in which R³ has the meaning indicated above.

The media preferably comprise one or more compounds of the formula III-2b, preferably selected from the group of the compounds of the formulae III-2b-1 and III-2b-2 and preferably of the formula III-2b-2:

in which R³ has the meaning indicated above.

The media preferably comprise one or more compounds of the formula III-2c, preferably selected from the group of the compounds of the formulae III-2c-1 to III-2c-5:

in which R³ has the meaning indicated above.

The media preferably comprise one or more compounds selected from the group of the compounds of the formulae III-2d and III-2e, preferably selected from the group of the compounds of the formulae III-2d-1 and III-2e-1:

in which R³ has the meaning indicated above.

The media preferably comprise one or more compounds of the formula III-2f, preferably selected from the group of the compounds of the formulae III-2f-1 to III-2f-5:

in which R³ has the meaning indicated above.

The media preferably comprise one or more compounds of the formula III-2g, preferably selected from the group of the compounds of the formulae III-2g-1 to III-2g-5:

in which R³ has the meaning indicated above.

The media preferably comprise one or more compounds of the formula III-2h, preferably selected from the group of the compounds of the formulae III-2h-1 to III-2h-5:

in which R³ has the meaning indicated above.

The media preferably comprise one or more compounds of the formula III-2i, preferably selected from the group of the compounds of the formulae III-2i-1 and III-2i-2:

in which R³ has the meaning indicated above.

The media preferably comprise one or more compounds of the formula III-2j, preferably selected from the group of the compounds of the formulae III-2j-1 and III-2j-2:

in which R³ has the meaning indicated above.

Alternatively or in addition to the compounds of the formulae III-1 and/or III-2, the media according to the present invention may comprise one or more compounds of the formula III-3

in which the parameters have the respective meanings indicated above under formula III and where the compounds are preferably selected from the group of the formulae III-3a and III-3b:

in which R³ has the meaning indicated above.

The liquid-crystalline media according to the present invention preferably comprise a dielectrically neutral component, component B. This component has a dielectric anisotropy in the range from −1.5 to 3. It preferably comprises, more preferably predominantly consists of, even more preferably essentially consists of and especially preferably entirely consists of dielectrically neutral compounds having a dielectric anisotropy in the range from −1.5 to 3. This component preferably comprises, more preferably predominantly consists of, even more preferably essentially consists of and very preferably entirely consists of one or more dielectrically neutral compounds having a dielectric anisotropy in the range from −1.5 to 3, of the formula IV.

The dielectrically neutral component, component B, preferably comprises one or more compounds selected from the group of the compounds of the formulae IV-1 to IV-6:

in which R⁴¹ and R⁴² have the respective meanings indicated above under formula IV, and in formulae IV-1, IV-5 and IV-6 R⁴¹ preferably denotes alkyl or alkenyl, preferably alkenyl, and R⁴² preferably denotes alkyl or alkenyl, preferably alkyl, in formula IV-2 R⁴¹ and R⁴² preferably denote alkyl, and in formula IV-4 R⁴¹ preferably denotes alkyl or alkenyl, more preferably alkyl, and R⁴² preferably denotes alkyl or alkoxy, more preferably alkoxy.

The dielectrically neutral component, component B, preferably comprises one or more compounds selected from the group of the compounds of the formulae IV-1, IV-4, IV-5 and IV-6, preferably one or more compounds of the formula IV-1 and one or more compounds selected from the group of the formulae IV-4 and IV-5, more preferably one or more compounds of each of the formulae IV-1, IV-4 and IV-5 and very preferably one or more compounds of each of the formulae IV-1, IV-4, IV-5 and IV-6.

In a preferred embodiment, component B preferably comprises one or more compounds of the formula IV-5, more preferably selected from the respective sub-formulae thereof of the formulae CCP-V-n and/or CCP-nV-m and/or CCP-Vn-m, more preferably of the formulae CCP-V-n and/or CCP-V2-n and very preferably selected from the group of the formulae CCP-V-1 and CCP-V2-1. The definitions of these abbreviations (acronyms) are indicated below in Table D or are evident from Tables A to C.

In a likewise preferred embodiment, component B preferably comprises one or more compounds of the formula IV-1, more preferably selected from the respective sub-formulae thereof of the formulae CC-n-m, CC-n-V and/or CC-n-Vm, more preferably of the formulae CC-n-V and/or CC-n-Vm and very preferably selected from the group of the formulae CC-3-V, CC-4-V, CC-5-V, CC-3-V1, CC-4-V1, CC-5-V1 and CC-3-V2. The definitions of these abbreviations (acronyms) are likewise indicated below in Table D or are evident from Tables A to C.

In a preferred embodiment, the concentration of the compound of the formula CC-3-V in the media according to the invention can be 50% to 65%, particularly preferably 55% to 60%.

In a further preferred embodiment of the present invention, which may be the same as the previous one or a different one, component B of the liquid-crystal mixtures according to the present invention, additionally or alternatively, preferably additionally, to one or more compounds of the formula IV selected from the group of the compounds of the formulae IV-1 to IV-6 as shown above, comprises one or more compounds of the formula IV selected from the group of the compounds of the formulae IV-7 to IV-14:

in which

-   R⁴¹ and R⁴², independently of one another, denote alkyl, alkoxy,     fluorinated alkyl or fluorinated alkoxy having 1 to 7 C atoms,     alkenyl, alkenyloxy, alkoxyalkyl or fluorinated alkenyl having 2 to     7 C atoms, and -   L⁴ denotes H or F.

In a preferred embodiment, component B preferably comprises one or more compounds of the formula IV-7, more preferably selected from the respective sub-formulae thereof of the formulae CPP-3-2, CPP-5-2 and CGP-3-2, more preferably of the formulae CPP-3-2 and/or CGP-3-2 and very particularly preferably of the formula CPP-3-2. The definitions of these abbreviations (acronyms) are indicated below in Table D or are evident from Tables A to C.

Alternatively or in addition to the compounds of the formulae II and/or III, component A of the media according to the present invention may comprise one or more dielectrically positive compounds of the formula V

in which

-   R⁵ denotes alkyl, alkoxy, fluorinated alkyl or fluorinated alkoxy     having 1 to 7 C atoms, alkenyl, alkenyloxy, alkoxyalkyl or     fluorinated alkenyl having 2 to 7 C atoms and preferably denotes     alkyl or alkenyl,

-    independently of one another, denote

-   L⁵¹ and L⁵², independently of one another, denote H or F, L⁵¹     preferably denotes F, and -   X⁵ denotes halogen, halogenated alkyl or alkoxy having 1 to 3 C     atoms or halogenated alkenyl or alkenyloxy having 2 or 3 C atoms,     preferably F, Cl, —OCF₃ or —CF₃, very preferably F, Cl or —OCF₃, -   Z⁵ denotes —CH₂CH₂—, —CF₂CF₂—, —COO—, trans-CH═CH—, trans-CF═CF— or     —CH₂O—, preferably —CH₂CH₂—, —COO— or trans-CH═CH— and very     preferably —COO— or trans-CH═CH—, and -   q denotes 0 or 1.

The media according to the present invention preferably comprise one or more compounds of the formula V, preferably selected from the group of the compounds of the formulae V-1 and V-2:

in which the parameters have the respective meanings indicated above, and the parameters L⁵³ and L⁵⁴, independently of one another and of the other parameters, denote H or F, and Z⁵ preferably denotes —CH₂—CH₂—.

The compounds of the formula V-1 are preferably selected from the group of the compounds of the formulae V-1a and V-1b:

in which R⁵ has the meaning indicated above.

The compounds of the formula V-2 are preferably selected from the group of the compounds of the formulae V-2a to V-2d:

in which R⁵ has the meaning indicated above.

The liquid-crystalline media according to the present invention preferably comprise, in the dielectrically neutral component, component B, one or more compounds of the formula VI

in which

-   R⁶¹ and R⁶², independently of one another, have the meaning     indicated above for R² under formula II, preferably R⁶¹ denotes     alkyl and R⁶² denotes alkyl or alkenyl,

-    and if it occurs twice, independently of one another on each     occurrence, denotes

-   -   preferably one or more of

-   -    denote

-   Z⁶¹ and Z⁶², independently of one another and, in the case where Z⁶¹     occurs twice, also these independently of one another, denote     —CH₂CH₂—, —COO—, trans-CH═CH—, trans-CF═CF—, —CH₂O—, —CF₂O— or a     single bond, preferably one or more of them denote(s) a single bond,     and -   r denotes 0, 1 or 2, preferably 0 or 1.

The liquid-crystalline media according to the present invention preferably comprise one or more compounds selected from the group of the compounds of the formulae VI-1 and VI-2:

in which R⁶¹ and R⁶² have the respective meanings indicated above under formula VI, and R⁶¹ preferably denotes alkyl, and in formula VI-1 R⁶² preferably denotes alkenyl, preferably —(CH₂)₂—CH═CH—CH₃, and in formula VI-2 R⁶² preferably denotes alkyl.

The liquid-crystalline media according to the present invention preferably comprise one or more compounds selected from the group of the compounds of the formulae VI-1 and VI-2, in which R⁶¹ preferably denotes n-alkyl, and in formula VI-1 R⁶² preferably denotes alkenyl, and in formula VI-2 R⁶² preferably denotes n-alkyl.

In a preferred embodiment, the media according to the present invention comprise one or more compounds of the formula VI-1, more preferably of the sub-formula PP-n-2Vm thereof, even more preferably of the formula PP-1-2V1. The definitions of these abbreviations (acronyms) are indicated below in Table D or are evident from Tables A to C.

In a preferred embodiment, the media according to the present invention comprise one or more compounds of the formula VI-2, more preferably of the sub-formula PGP-n-m thereof, even more preferably of the sub-formula PGP-3-m thereof, very preferably selected from the formulae PGP-3-2, PGP-3-3, PGP-3-4 and PGP-3-5. The definitions of these abbreviations (acronyms) are likewise indicated below in Table D or are evident from Tables A to C.

In addition, the liquid-crystal mixtures according to the present invention may comprise a further optional component, component C, which has negative dielectric anisotropy and comprises, preferably predominantly consists of, more preferably essentially consists of and very preferably entirely consists of dielectrically negative compounds, preferably of the formula VII

in which

-   R⁷¹ and R⁷², independently of one another, have the meaning     indicated above for R² under formula II,

-    denotes

-   -   preferably

-    denotes

-   Z⁷¹ and Z⁷², independently of one another, denote —CH₂CH₂—, —COO—,     trans-CH═CH—, trans-CF═CF—, —CH₂O—, —CF₂O— or a single bond,     preferably one or more of them denote(s) a single bond and very     preferably both denote a single bond, -   L⁷¹ and L⁷², independently of one another, denote C—F or N,     preferably one or more of them denote(s) C—F and very preferably     both denote C—F, and -   s denotes 0 or 1.

In addition to the compounds already mentioned, the liquid-crystal mixtures according to the present invention may comprise, in component A, one or more compounds of the formula VIII

in which

-   R⁸ has the meaning indicated above for R² under formula II, -   one of

-    which is present denotes

-    preferably

-   -   preferably

-    denotes

and the others have the same meaning or, independently of one another, denote

-    preferably

-   Z⁸¹ and Z⁸², independently of one another, denote —CH₂CH₂—, —COO—,     trans-CH═CH—, trans-CF═CF—, —CH₂O—, —CF₂O— or a single bond,     preferably one or more of them denote(s) a single bond and very     preferably both denote a single bond, -   t denotes 0, 1 or 2, preferably 0 or 1, more preferably 1, and -   X⁸ has the meaning indicated above for X² under formula II or     alternatively, independently of R⁸, may have the meaning indicated     for R⁸, and     from which the compounds of the formula I are excluded.

In this application, comprise in connection with compositions means that the relevant entity, i.e. the medium or the component, comprises the component or components or the compound or compounds indicated, preferably in a total concentration of 10% or more and very preferably 20% or more.

In this connection, predominantly consist of means that the relevant entity comprises 55% or more, preferably 60% or more and very preferably 70% or more of the component or components or the compound or compounds indicated.

In this connection, essentially consist of means that the relevant entity comprises 80% or more, preferably 90% or more and very preferably 95% or more of the component or components or the compound or compounds indicated.

In this connection, entirely consist of means that the relevant entity comprises 98% or more, preferably 99% or more and very preferably 100.0% of the component or components or the compound or compounds indicated.

Component C preferably comprises, more preferably predominantly consists of and very preferably entirely consists of one or more compounds of the formula VII, preferably selected from the group of the compounds of the formulae VII-1 to VII-3:

in which

-   R⁷¹ and R⁷² have the respective meanings indicated above under     formula VII.

In formulae VII-1 to VII-3, R⁷¹ preferably denotes n-alkyl or 1E-alkenyl and R⁷² preferably denotes n-alkyl or alkoxy.

Other mesogenic compounds which are not explicitly mentioned above can optionally and advantageously also be used in the media according to the present invention. Such compounds are known to the person skilled in the art.

The liquid-crystal media according to the present invention preferably have a clearing point of 60° C. or more, more preferably 65° C. or more, particularly preferably 70° C. or more and very particularly preferably 75° C. or more.

The Δn of the liquid-crystal media according to the present invention, at 589 nm (Na^(D)) and 20° C., is preferably in the range from 0.060 or more to 0.140 or less, more preferably in the range from 0.070 or more to 0.130 or less, even more preferably in the range from 0.080 or more to 0.125 or less and very preferably in the range from 0.090 or more to 0.122 or less.

In a preferred embodiment of the present application, the Δn of the liquid-crystal media according to the present invention is preferably 0.080 or more, more preferably 0.090 or more and in a particular embodiment preferably 0.100 or more.

The Δε of the liquid-crystal medium according to the invention, at 1 kHz and 20° C., is preferably 2 or more, more preferably 3 or more and in a particularly preferred embodiment 4 or more and very preferably 6 or more. In particular, Δε is 15 or less.

The nematic phase of the media according to the invention preferably extends at least from 0° C. or less to 70° C. or more, more preferably at least from −20° C. or less to 70° C. or more, very preferably at least from −30° C. or less to 75° C. or more and in particular at least from −40° C. or less to 75° C. or more.

In a first preferred embodiment of the present invention, the Δn of the liquid-crystal media is in the range from 0.090 or more to 0.130 or less, more preferably in the range from 0.095 or more to 0.120 or less and very preferably in the range from 0.100 or more to 0.115 or less, while Δε is preferably in the range from 2 or more to 12 or less, preferably 10 or less.

In a second preferred embodiment of the present invention, the Δn of the liquid-crystal media is in the range from 0.085 or more to 0.130 or less, more preferably in the range from 0.090 or more to 0.125 or less and very preferably in the range from 0.095 or more to 0.120 or less, while Δε is preferably 4 or more, more preferably 6 or more, even more preferably 8.0 or more and very preferably in the range from 8.0 or more to 10.0 or less.

In this embodiment, the nematic phase of the media according to the invention preferably extends at least from −20° C. or less to 70° C. or more, more preferably at least from −20° C. or less to 70° C. or more, very preferably at least from −30° C. or less to 70° C. or more and in particular at least from −40° C. or less to 70° C. or more.

In a third preferred embodiment of the present invention, the Δn of the liquid-crystal media is in the range from 0.070 or more to 0.120 or less, more preferably in the range from 0.075 or more to 0.115 or less and very preferably in the range from 0.080 or more to 0.110 or less, while Δε is preferably 2.0 or more, more preferably in the range from 3.0 or more to 14.0 or less and very preferably either in the range from 4.0 or more to 6.0 or less or particularly preferably in the range from 6.0 or more to 11.0 or less.

In this embodiment, the nematic phase of the media according to the invention preferably extends at least from −20° C. or less to 75° C. or more, more preferably at least from −30° C. or less to 70° C. or more, very preferably at least from −30° C. or less to 75° C. or more and in particular at least from −30° C. or less to 80° C. or more.

In a fourth preferred embodiment of the present invention, the Δn of the liquid-crystal media is in the range from 0.080 or more to 0.120 or less, more preferably in the range from 0.085 or more to 0.115 or less and very preferably in the range from 0.090 or more to 0.110 or less, while Δε is preferably 1.5 or more, more preferably in the range from 2.0 or more to 8.0 or less and very preferably either in the range from 2.0 or more to 6.0 or less or particularly preferably in the range from 2.0 or more to 4.0 or less.

In this embodiment, the nematic phase of the media according to the invention preferably extends at least from −20° C. or less to 65° C. or more, more preferably at least from −30° C. or less to 70° C. or more, very preferably at least from −30° C. or less to 75° C. or more and in particular at least from −30° C. or less to 80° C. or more.

In a fifth preferred embodiment of the present invention, the Δn of the liquid-crystal media is in the range from 0.110 or more to 0.150 or less, more preferably in the range from 0.120 or more to 0.140 or less and very preferably in the range from 0.125 or more to 0.135 or less, while Δε is preferably 2.0 or more, more preferably in the range from 2.5 or more to 8.0 or less and very preferably in the range from 3.0 or more to 7.0 or less.

In this embodiment, the nematic phase of the media according to the invention preferably extends at least from −20° C. or less to 70° C. or more, more preferably at least from −30° C. or less to 70° C. or more, very preferably at least from −20° C. or less to 75° C. or more and in particular at least from −30° C. or less to 75° C. or more.

In a sixth preferred embodiment of the present invention, the Δn of the liquid-crystal media is in the range from 0.110 or more to 0.150 or less, more preferably in the range from 0.120 or more to 0.145 or less and very preferably from about 0.137 or more to 0.100 or less, preferably in the range from 2.0 or more to 8.0 or less and very preferably in the range from 3.0 or more to 7.0 or less.

In this embodiment, the nematic phase of the media according to the invention preferably extends at least from −20° C. or less to 70° C. or more, more preferably at least from −30° C. or less to 70° C. or more, very preferably at least from −30° C. or less to 75° C. or more and in particular at least from −30° C. or less to 80° C. or more.

In accordance with the present invention, component A is preferably used in a concentration of 1% to 50%, more preferably 1% to 30%, more preferably 2% to 30% and very preferably 3% to 30% of the mixture as a whole.

The compounds of the formulae II and III are preferably used in a total concentration of 2% to 60%, more preferably 3% to 55%, even more preferably 10% to 40%, even more preferably 15% to 35% and very preferably 25% to 30% of the mixture as a whole.

The compounds of the formula IV are preferably used in a total concentration of 0% to 70%, more preferably 10%, preferably 15 to 65%, even more preferably 15%, preferably 20% to 60% and very preferably 15% to 55% of the mixture as a whole.

The compounds of the formula V are preferably used in a total concentration of 0% to 50%, more preferably 1% to 40%, even more preferably 5% to 30% and very preferably 7% to 25% of the mixture as a whole.

Component C is preferably used in a concentration of 0% to 30%, more preferably 0% to 15% and very preferably 1% to 10% of the mixture as a whole.

The media according to the invention may optionally comprise further liquid-crystal compounds in order to adjust the physical properties. Such compounds are known to the person skilled in the art. Their concentration in the media according to the present invention is preferably 0% to 30%, more preferably 0.1% to 20% and very preferably 1% to 15%.

In the first preferred embodiment of the present invention mentioned above, the compounds of the formula I are preferably used in a total concentration of 1% to 30%, more preferably 3% to 25% and very preferably 5% to 20% of the mixture as a whole, while the compounds of the formula VI are preferably used in a concentration of 5% to 40%, more preferably 8% to 35% and very preferably 10% to 30% of the mixture as a whole.

In this preferred embodiment, the media preferably comprise one or more compounds of the formula VI and very preferably of the formula VI-2.

Especially in the second preferred embodiment of the present invention mentioned above, component B preferably comprises one or more compounds of the formula IV, more preferably of the formula IV-1, even more preferably selected from the respective sub-formulae thereof of the formulae CC-n-V and/or CC-n-Vm, more preferably of the formulae CC-n-V1 and/or CC-n-V and very preferably selected from the group of the formulae CC-3-V, CC-4-V, CC-5-V and CC-3-V1. The definitions of these abbreviations (acronyms) are indicated below in Table D.

The liquid-crystal media preferably comprise in total 50% to 100%, more preferably 70% to 100% and very preferably 80% to 100% and in particular 90% to 100% of one or more compounds selected from the group of the compounds of the formulae I, II, III, IV, V, VI, VII and VIII, preferably of the formulae I, II, III, IV, V, VI and VII and particularly preferably of the formulae I, II, III, IV, V and VI.

In the present application, the expression dielectrically positive describes compounds or components where Δε>3.0, dielectrically neutral describes those where −1.5≦Δε≦3.0 and dielectrically negative describes those where Δε<−1.5. Δε is determined at a frequency of 1 kHz and at 20° C. The dielectric anisotropy of the respective compound is determined from the results of a solution of 10% of the respective individual compound in a nematic host mixture. If the solubility of the respective compound in the host mixture is less than 10%, the concentration is reduced to 5%. The capacitances of the test mixtures are determined both in a cell having homeotropic alignment and in a cell having homogeneous alignment. The cell thickness of both types of cells is approximately 20 μm. The voltage applied is a rectangular wave having a frequency of 1 kHz and an effective value of typically 0.5 V to 1.0 V, but it is always selected to be below the capacitive threshold of the respective test mixture.

Δε is defined as (ε_(∥)−Ε_(⊥)), while ε_(av.) is (ε_(∥)+2ε_(⊥))/3.

The host mixture used for dielectrically positive compounds is mixture ZLI-4792 and that used for dielectrically neutral and dielectrically negative compounds is mixture ZLI-3086, both from Merck KGaA, Germany. The dielectric permittivities of the compounds are determined from the change in the respective values of the host mixture on addition of the compounds of interest. The values are extrapolated to a concentration of the compounds of interest of 100%.

Components having a nematic phase at the measurement temperature of 20° C. are measured as such, all others are treated like compounds.

The expression threshold voltage in the present application refers to the optical threshold and is quoted for 10% relative contrast (V₁₀), and the expression saturation voltage refers to the optical saturation and is quoted for 90% relative contrast (V₉₀), in both cases unless expressly stated otherwise. The capacitive threshold voltage (V₀), also called the Freedericksz threshold (V_(Fr)), is only used if expressly mentioned.

The ranges of the parameters indicated in this application all include the limit values, unless expressly stated otherwise.

The different upper and lower limit values indicated for various ranges of properties in combination with one another give rise to additional preferred ranges.

Throughout this application, the following conditions and definitions apply, unless expressly stated otherwise. All concentrations are indicated in percent by weight and relate to the respective mixture as a whole, all temperatures are quoted in degrees Celsius and all temperature differences are quoted in differential degrees. All physical properties are determined in accordance with “Merck Liquid Crystals, Physical Properties of Liquid Crystals”, Status November 1997, Merck KGaA, Germany and are quoted for a temperature of 20° C., unless expressly stated otherwise. The optical anisotropy (Δn) is determined at a wavelength of 589.3 nm. The dielectric anisotropy (Δε) is determined at a frequency of 1 kHz. The threshold voltages, as well as all other electro-optical properties, are determined using test cells produced at Merck KGaA, Germany. The test cells for the determination of Δε have a cell thickness of approximately 20 μm. The electrode is a circular ITO electrode having an area of 1.13 cm² and a guard ring. The orientation layers are SE-1211 from Nissan Chemicals, Japan, for homeotropic orientation (ε_(∥)) and polyimide AL-1054 from Japan Synthetic Rubber, Japan, for homogeneous orientation (ε_(⊥)). The capacitances are determined using a Solatron 1260 frequency response analyser using a sine wave with a voltage of 0.3 V_(rms). The light used in the electro-optical measurements is white light. A set-up using a commercially available DMS instrument from Autronic-Melchers, Germany, is used here. The characteristic voltages have been determined under perpendicular observation. The threshold (V₁₀), mid-grey (V₅₀) and saturation (V₉₀) voltages have been determined for 10%, 50% and 90% relative contrast, respectively.

The liquid-crystal media according to the present invention may comprise further additives and chiral dopants in the usual concentrations. The total concentration of these further constituents is in the range from 0% to 10%, preferably 0.1% to 6%, based on the mixture as a whole. The concentrations of the individual compounds used are each preferably in the range from 0.1% to 3%. The concentration of these and similar additives is not taken into consideration when quoting the values and concentration ranges of the liquid-crystal components and compounds of the liquid-crystal media in this application.

The liquid-crystal media according to the invention consist of a plurality of compounds, preferably 3 to 30, more preferably 4 to 20 and very preferably 4 to 16 compounds. These compounds are mixed in a conventional manner. In general, the desired amount of the compound used in the smaller amount is dissolved in the compound used in the larger amount. If the temperature is above the clearing point of the compound used in the higher concentration, it is particularly easy to observe completion of the dissolution process. It is, however, also possible to prepare the media in other conventional ways, for example using so-called pre-mixes, which can be, for example, homologous or eutectic mixtures of compounds, or using so-called “multibottle” systems, the constituents of which are themselves ready-to-use mixtures.

By addition of suitable additives, the liquid-crystal media according to the present invention can be modified in such a way that they can be used in all known types of liquid-crystal displays, either using the liquid-crystal media as such, such as TN, TN-AMD, ECB-AMD, VAN-AMD, IPS-AMD, FFS-AMD LCDs, or in composite systems, such as PDLC, NCAP, PN LCDs and especially in ASM-PA LCDs.

All temperatures, such as, for example, the melting point T(C,N) or T(C,S), the transition from the smectic (S) to the nematic (N) phase T(S,N) and the clearing point T(N,I) of the liquid crystals, are quoted in degrees Celsius. All temperature differences are quoted in differential degrees.

In the present invention and especially in the following examples, the structures of the mesogenic compounds are indicated by means of abbreviations, also called acronyms. In these acronyms, the chemical formulae are abbreviated as follows using Tables A to C below. All groups C_(n)H_(2n+1), C_(m)H_(2m+1) and C_(l)H_(2l+1) or C_(n)H_(2n−1), C_(m)H_(2m−1) and C_(l)H_(2l−1) denote straight-chain alkyl or alkenyl, preferably 1E-alkenyl, each having n, m and I C atoms respectively. Table A lists the codes used for the ring elements of the core structures of the compounds, while Table B shows the linking groups. Table C lists the meanings of the codes for the left-hand or right-hand end groups. Table D shows illustrative structures of compounds together with their respective abbreviations.

TABLE A Ring elements C

D

A

G

U

Y

M

N

(NF)

Np

N3f

tH

K

L

F

P

DI

AI

GI

UI

MI

NI

(NF)I

dH

N3fI

tHI

KI

LI

FI

TABLE B Linking groups E —CH₂CH₂— Z —CO—O— V —CH═CH— ZI —O—CO— X —CF═CH— O —CH₂—O— XI —CH═CF— OI —O—CH₂— B —CF═CF— Q —CF₂—O— T —C≡C— QI —O—CF₂— W —CF₂CF₂— T —C≡C—

TABLE C End groups Left-hand side Right-hand side Use alone -n- C_(n)H_(2n+1)— -n —C_(n)H_(2n+1) -nO- C_(n)H_(2n+1)—O— -nO —O—C_(n)H_(2n+1) -V- CH₂═CH— -V —CH═CH₂ -nV- C_(n)H_(2n+1)—CH═CH— -nV —CnH_(2n)—CH═CH₂ -Vn- CH₂═CH—C_(n)H_(2n+1)— -Vn —CH═CH—C_(n)H_(2n+1) -nVm- C_(n)H_(2n+1)—CH═CH—C_(m)H_(2m)— -nVm —C_(n)H_(2n)—CH═CH—C_(m)H_(2m+1) -N- N≡C— -N —C≡N -S- S═C═N— -S —N═C═S -F- F— -F —F -CL- Cl— -CL —Cl -M- CFH₂— -M —CFH₂ -D- CF₂H— -D —CF₂H -T- CF₃— -T —CF₃ -MO- CFH₂O— -OM —OCFH₂ -DO- CF₂HO— -OD —OCF₂H -TO- CF₃O— -OT —OCF₃ -OXF- CF₂═CH—O— -OXF —O—CH═CF₂ -A- H—C≡C— -A —C≡C—H -nA- C_(n)H_(2n+1)—C≡C— -An —C≡C—C_(n)H_(2n+1) -NA- N≡C—C≡C— -AN —C≡C—C≡N Use in combination with further -...A...- —C≡C— -...A... —C≡C— -...V...- CH═CH— -...V... —CH═CH— -...Z...- —CO—O— -...Z... —CO—O— -...ZI...- —O—CO— -...ZI... —O—CO— -...K...- —CO— -...K... —CO— -...W...- —CF═CF— -...W... —CF═CF— in which n and m each denote integers, and the three dots “...” are spacers for other abbreviations from this table.

The following table shows illustrative structures together with their respective abbreviations. These are shown in order to illustrate the meaning of the rules for the abbreviations. They furthermore represent compounds which are preferably used.

TABLE D Illustrative structures

The following table, Table E, shows illustrative compounds which can be used as stabiliser in the mesogenic media according to the present invention.

TABLE E

In a preferred embodiment of the present invention, the mesogenic media comprise one or more compounds selected from the group of the compounds from Table E.

The following table, Table F, shows illustrative compounds which can preferably be used as chiral dopants in the mesogenic media according to the present invention.

TABLE F

In a preferred embodiment of the present invention, the mesogenic media comprise one or more compounds selected from the group of the compounds from Table F.

The mesogenic media according to the present application preferably comprise two or more, preferably four or more, compounds selected from the group consisting of the compounds from the above tables.

The liquid-crystal media according to the present invention preferably comprise

-   -   seven or more, preferably eight or more, compounds, preferably         compounds having three or more, preferably four or more,         different formulae, selected from the group of the compounds         from Table D.

EXAMPLES

The examples below illustrate the present invention without limiting it in any way.

However, the physical properties show the person skilled in the art what properties can be achieved and in what ranges they can be modified. In particular, the combination of the various properties which can preferably be achieved is thus well defined for the person skilled in the art.

Example 1

A liquid-crystal mixture having the composition and properties as indicated in the following table is prepared.

Composition Compound No. Abbreviation 1 P(NF)P-2-4 8.0 2 PGU-3-F 5.0 3 PUQU-3-F 4.0 4 APUQU-3-F 9.0 5 CC-3-V 40.0 6 CC-3-V1 7.0 7 PP-1-2V1 6.0 8 CCP-V-1 6.0 9 PGP-2-3 5.0 10  PGP-2-4 5.0 11  PGP-2-5 5.0 Σ 100.0 Physical properties T(N, I) = 73° C. n_(e) (20° C., 589.3 nm) = 1.6221 Δn (20° C., 589.3 nm) = 0.1280 ε_(||) (20° C., 1 kHz) = 7.2 Δε (20° C., 1 kHz) = 4.3 γ₁ (20° C.) = 52 mPa · s V₁₀ (20° C.) = 2.06 V V₉₀ (20° C.) = 2.99 V V

This mixture is very highly suitable for displays in TN mode and in particular for displays for use as monitor for a PC.

Example 2

A liquid-crystal mixture having the composition and properties as indicated in the following table is prepared.

Composition Compound No. Abbreviation 1 P(NF)P-2-4 8.0 2 PGU-3-F 9.0 3 PUQU-3-F 3.0 4 APUQU-3-F 8.0 5 CC-3-V 36.0 6 CC-3-V1 7.0 7 PP-1-2V1 5.0 8 CCP-V-1 6.0 9 PGP-2-3 5.0 10  PGP-2-4 6.0 11  PGP-2-5 7.0 Σ 100.0 Physical properties T(N, I) = 76° C. n_(e) (20° C., 589.3 nm) = 1.6350 Δn (20° C., 589.3 nm) = 0.1384 ε_(||) (20° C., 1 kHz) = 7.8 Δε (20° C., 1 kHz) = 4.8 γ₁ (20° C.) = 59 mPa · s V₁₀ (20° C.) = 2.03 V V₉₀ (20° C.) = 2.94 V

This mixture is very highly suitable for displays in TN mode and in particular for displays for use as PC monitor.

Example 3

A liquid-crystal mixture having the composition and properties as indicated in the following table is prepared.

Composition Compound No. Abbreviation 1 P(NF)P-2-4 7.0 2 PGU-2-F 2.5 3 PUQU-3-F 15.0 4 CCGU-3-F 1.5 5 APUQU-2-F 6.0 6 CC-3-V 35.0 7 CC-3-V1 11.5 8 CCP-V-1 11.5 9 CCP-V2-1 3.0 10  PGP-2-3 7.0 Σ 100.0 Physical properties T(N, I) = 74.5° C. n_(e) (20° C., 589.3 nm) = 1.6006 Δn (20° C., 589.3 nm) = 0.1120 ε_(||) (20° C., 1 kHz) = 9.0 Δε (20° C., 1 kHz) = 6.1 γ₁ (20° C.) = 55 mPa · s k₁ (20° C.) = 12.9 pN k₃/k₁ (20° C.) = 0.99 V₀ (20° C.) = 1.56 V

This mixture is very highly suitable for displays in IPS mode and in particular for displays for use as PC monitor.

Example 4

A liquid-crystal mixture having the composition and properties as indicated in the following table is prepared.

Composition Compound No. Abbreviation 1 P(NF)P-2-4 7.5 2 PGU-2-F 2.5 3 PUQU-3-F 19.5 4 CCGU-3-F 6.0 5 CC-3-V 43.0 6 CC-3-V1 6.0 7 CCP-V-1 3.0 8 CPP-3-2 10.0 9 CPPC-3-3 2.5 Σ 100.0 Physical properties T(N, I) = 73.5° C. n_(e) (20° C., 589.3 nm) = 1.5962 Δn (20° C., 589.3 nm) = 0.1088 ε_(||) (20° C., 1 kHz) = 8.9 Δε (20° C., 1 kHz) = 5.8 γ₁ (20° C.) = 51 mPa · s

This mixture is very highly suitable for displays in IPS mode and in particular for displays for use as PC monitor.

Example 5

A liquid-crystal mixture having the composition and properties as indicated in the following table is prepared.

Composition Compound No. Abbreviation 1 P(NF)P-2-4 7.0 2 PGU-2-F 8.0 3 PUQU-2-F 9.0 4 PUQU-3-F 9.0 5 CC-3-V 33.0 6 CC-3-V1 13.0 7 CCP-V-1 13.0 8 CPP-3-2 8.0 Σ 100.0 Physical properties T(N, I) = 69° C. n_(e) (20° C., 589.3 nm) = 1.5990 Δn (20° C., 589.3 nm) = 0.1095 ε_(||) (20° C., 1 kHz) = 8.8 Δε (20° C., 1 kHz) = 5.8 γ₁ (20° C.) = 47 mPa · s k₁ (20° C.) = 12.2 pN k₃/k₁ (20° C.) = 0.94 V₀ (20° C.) = 1.52 V

This mixture is very highly suitable for displays in IPS mode and in particular for displays for use as PC monitor. 

1. Liquid-crystal medium, characterised in that it comprises one or more compounds of the formula I

in which R¹¹ and R¹², independently of one another, denote alkyl, alkoxy, fluorinated alkyl or fluorinated alkoxy having 1 to 7 C atoms, alkenyl, alkenyloxy, alkoxyalkyl or fluorinated alkenyl, alkenyloxy or alkoxyalkyl having 2 to 7 C atoms.
 2. Liquid-crystal medium according to claim 1, characterised in that it comprises one or more compounds selected from the group of the compounds of the formulae II and III

in which R² and R³, independently of one another, denote alkyl, alkoxy, fluorinated alkyl or fluorinated alkoxy having 1 to 7 C atoms, alkenyl, alkenyloxy, alkoxyalkyl or fluorinated alkenyl having 2 to 7 C atoms,

independently of one another, denote

L²¹, L²², L³¹ and L³², independently of one another, denote H or F, X² and X³, independently of one another, denote halogen, halogenated alkyl or alkoxy having 1 to 3 C atoms or halogenated alkenyl or alkenyloxy having 2 or 3 C atoms, Z³ denotes —CH₂CH₂—, —CF₂CF₂—, —COO—, trans-CH═CH—, trans-CF═CF—, —CH₂O— or a single bond, and l, m, n and o, independently of one another, denote 0 or
 1. 3. Liquid-crystal medium according to claim 1, characterised in that it comprises one or more compounds of the formula IV

in which R⁴¹ and R⁴², independently of one another, denote alkyl, alkoxy, fluorinated alkyl or fluorinated alkoxy having 1 to 7 C atoms, alkenyl, alkenyloxy, alkoxyalkyl or fluorinated alkenyl baying 2 to 7 C atoms,

independently of one another and, in the case where

 occurs twice, also these independently of one another, denote

Z⁴¹ and Z⁴², independently of one another and, in the case where Z⁴¹ occurs twice, also these independently of one another, denote —CH₂CH₂—, —COO—, trans-CR═CH—, trans-CF═CF—, —CH₂O—, —CF₂O—, —C≡C— or a single bond, and p denotes 0, 1 or
 2. 4. Liquid-crystal medium according to claim 1, characterised in that the total concentration of the compounds of the formula I in the medium is in the range from 1% to 30%.
 5. Liquid-crystal medium according to claim 1, characterised in that the medium comprises one or more compounds of the formula I in which R¹¹ and R¹², independently of one another, denote alkyl.
 6. Liquid-crystal medium according to claim 2, characterised in that it comprises one or more compounds of the formula II.
 7. Liquid-crystal medium according to claim 2, characterised in that it comprises one or more compounds of the formula III.
 8. Liquid-crystal medium according to claim 2, characterised in that it comprises one or more compounds of the formula VI

in which R⁶¹ and R⁶², independently of one another, have the meaning indicated for R² in claim 2,

 and if it occurs twice, independently of one another on each occurrence, denotes

Z⁶¹ and Z⁶², independently of one another and, in the case where Z⁶¹ occurs twice, also these independently of one another, denote —CH₂CH₂—, —COO—, trans-CH═CH—, trans-CF═CF—, —CH₂O—, —CF₂O— or a single bond, and r denotes 0, 1 or
 2. 9. Liquid-crystal display, characterised in that it contains a liquid-crystal medium according to claim
 1. 10. Liquid-crystal display according to claim 9, characterised in that it is addressed by an active matrix.
 11. (canceled)
 12. Process for the preparation of a liquid-crystal medium, characterised in that one or more compounds of the formula I according to claim 1 are mixed with one or more further compounds and optionally with one or more additives. 